Method for adapting for an interoperability between short-term correlation models of digital signals

Abstract

The invention relates to the code conversion of digital signals, particularly voice signals, and in particular coding according to a second format from information obtained by carrying out a coding according to a first format. These first and second formats use LPC (linear predictive coding) short-term prediction models on digital signal sample blocks while using filters represented by respective LPC coefficients. The LPC coefficients of the second format are determined from an interpolation on the representative values of the LPC coefficients of at least the first format, between at least one given block and a preceding block. According to the invention, the interpolation (43), is dynamically effected while selecting (42), for each current block, at least one interpolation factor (α) among a preselection of factors according to a predetermined criterion such as a stationarity criterion of the digital signal (41).

1. A method performed by a coder for coding a digital speech signal according to a second format of coding, from information corresponding to a first format of coding, comprising:
 carrying out at least one coding step according to the first format of coding; 
 interpolating a value representative of a first plurality of linear predictive coding (LPC) coefficients corresponding to the first format, between a given block and a preceding block, which precedes the given block, to provide a second plurality of LPC coefficients corresponding to the second format, 
 wherein the first and second formats use, LPC short-term prediction models on digital speech signal sample blocks, by using filters represented by the respective first and second pluralities of LPC coefficients, 
 wherein the method further comprises:
 a step of detection of a break in stationarity of the digital speech signal at least between the given block and the preceding block, 
 a step of interpolation performed dynamically, by choosing for each current block at least one interpolation factor from a preselection of factors, according to said detection, and 
 a step of transforming said digital speech signal into a coded signal according to said second format of coding, using said LPC coefficients of the second format obtained by said interpolation, and wherein: 
 
 a moment of break in stationarity is detected between a given block and a preceding block 
 this break moment is compared with a time position of a current block in the second format, and, 
 in the interpolation, more weight is assigned to the LPC coefficients of the first format that are associated with the given block if the block of the second format occurs after the detected break moment, or to the LPC coefficients of the first format that are associated with the preceding block if the block of the second format occurs before the detected break moment. 
 
     
     
       2. The method as claimed in  claim 1 , wherein the preselection is initially set to include the value “0”, the value “1” and at least one third value between “0” and “1”. 
     
     
       3. The method as claimed in  claim 1 , wherein the number of elements in said preselection is chosen according to a predetermined trade-off between quality and complexity. 
     
     
       4. The method as claimed in  claim 1 , wherein said interpolation applies a first factor relating to said given block and a second factor relating to said preceding block, and the first and second factors are deduced from each other. 
     
     
       5. The method as claimed in  claim 4 , wherein the first factor, represented by β, and the second factor, represented by α, are between “0” and “1” and are deduced from each other by the relation α=1−β. 
     
     
       6. The method as claimed in  claim 1 , wherein the choice of an interpolation factor from said preselection of factors, at least for each current block, is performed before interpolation. 
     
     
       7. The method as claimed in  claim 6 , wherein a prior choice of an interpolation factor applies a prior classification based on a local stationarity criterion detected on the chosen parameters, obtained by the first coding format. 
     
     
       8. The method as claimed in  claim 1 , wherein the preselection is initially set following a preliminary statistical study. 
     
     
       9. The method as claimed in  claim 8 , wherein the statistical study comprises:
 respective sets of values representative of LPC coefficients obtained by the first format over a plurality of blocks, and of values representative of LPC coefficients obtained by the second format over a plurality of blocks; and 
 a first set of interpolation factors chosen to include said preselection, 
 wherein, for each block, from said first set, a revised interpolation factor is determined according to a chosen criterion, notably a distance between the interpolated values and the values representative of coefficients obtained by the second format, to obtain a smaller second set of interpolation factors. 
 
     
     
       10. The method as claimed in  claim 9 , wherein the step of determining the revised interpolation factor is repeated with said second set, then with other successive subsets, until said preselection is obtained. 
     
     
       11. A code conversion module, for coding a digital signal according to a second format, from information obtained by carrying out at least one coding of the signal according to a first format, the first and second formats using, for coding a speech signal, LPC short-term prediction models on digital signal sample blocks, by using filters represented by respective LPC coefficients, the module comprising:
 an input for receiving information representative of the LPC coefficients obtained by the first format; and 
 a processing unit for determining the LPC coefficients of the second format from an interpolation on values representative of the LPC coefficients obtained from the first format between at least one first block and a second block, preceding the first block, 
 wherein the processing unit performs a detection of a break in stationarity of the digital signal at least between the given block and the preceding block, and 
 the processing unit performs said interpolation dynamically, by choosing for each current block at least one interpolation factor from a preselection of factors, according to said detection, and wherein:
 the processing unit detects a moment of break in stationarity between a given block and a preceding block, 
 this break moment is compared with a time position of a current block in the second format, and, 
 in the interpolation, more weight is assigned to the LPC coefficients of the first format that are associated with the given block if the block of the second format occurs after the detected break moment, or to the LPC coefficients of the first format that are associated with the preceding block if the block of the second format occurs before the detected break moment. 
 
 
     
     
       12. A non-transitory computer storage medium storing a computer program product, designed to be stored in a memory of a code conversion module, to code a signal according to a second format, from information obtained by carrying out at least one coding of the same signal according to a first format, the first and second formats using, for coding a speech signal, LPC short-term prediction models on digital signal sample blocks, by using filters represented by respective LPC coefficients,
 the computer program comprising the steps of:
 determining values representative of the LPC coefficients of the second format from an interpolation on values representative of the LPC coefficients obtained from the first format between at least one first given block and a second block, preceding the first block; and 
 dynamically performing said interpolation, by choosing for each current block at least one interpolation factor from a preselection of factors, according to a detection of a break in stationarity of the digital signal at least between the given block and the preceding block, and wherein:
 a moment of break in stationarity is detected between a given block and a preceding block, 
 this break moment is compared with a time position of a current block in the second format, and, 
 
 
 in the interpolation, more weight is assigned to the LPC coefficients of the first format that are associated with the given block if the block of the second format occurs after the detected break moment, or to the LPC coefficients of the first format that are associated with the preceding block if the block of the second format occurs before the detected break moment. 
 
     
     
       13. A signal coding system for a speech signal, comprising:
 a coder according to a first format and a coder according to a second format, using LPC short-term prediction models on digital signal sample blocks, by using filters represented by respective LPC coefficients; and 
 a code conversion module for adapting the coding of the signal to the second format, from information obtained by carrying out the coding of the same signal according to the first format, wherein the module includes: 
 an input for receiving information representative of the LPC coefficients obtained by the first format; and 
 a processing unit for determining the LPC coefficients of the second format from an interpolation on values representative of the LPC coefficients obtained from the first format between at least one first given block and a second block, preceding the first block, wherein the processing unit performs a detection of a break in stationarity of the digital signal at least between the given block and the preceding block, and the processing unit performs said interpolation dynamically, by choosing for each current block at least one interpolation factor from a preselection of factors, according to said detection, and wherein:
 the processing unit detects a moment of break in stationarity between a given block and a preceding block, 
 this break moment is compared with a time position of a current block in the second format, and, 
 in the interpolation, more weight is assigned to the LPC coefficients of the first format that are associated with the given block if the block of the second format occurs after the detected break moment, or to the LPC coefficients of the first format that are associated with the preceding block if the block of the second format occurs before the detected break moment. 
 
 
     
     
       14. The system as claimed in  claim 13 , wherein said module is integrated in the coder according to the second format.